Propeller Including a Blade Back Flow Guide

ABSTRACT

THIS invention relates to a propeller and more particularly but not exclusively, to a propeller for use with inboard and outboard boat engines. The propeller includes a hub, and a plurality of blades, each blade having a blade face, a blade back and a blade peripheral zone comprising a leading edge, a trialing edge and a blade tip zone extending between the leading edge and the trialing edge. The propeller is characterized in that a flow guide is provided on the blade back of each blade.

BACKGROUND TO THE INVENTION

THIS invention relates to a propeller and more particularly but not exclusively, to a propeller for use with inboard and outboard boat engines.

A propeller is a device that transmits power by converting rotational motion into thrust. A pressure differential is produced between forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade, thus resulting in thrust required to drive a means of transport to which the propeller is attached. One specific type of propeller is a propeller for use as means of propulsion in boat engines, whether outboard or inboard.

Many different propeller designs are known in the trade, and they all share some of the same design characteristics. A propeller comprises a plurality of blades extending radially outwardly from a central rotating hub. Each blade is in the form of an airfoil having two opposite surfaces, being a blade face (which is the pressure side of the blade facing the stern), and the blade back (which is the suction side of the blade facing the bow). Each blade furthermore has a leading edge, which is the edge of the propeller adjacent the forward end of the hub. The leading edge leads the blade into the flow when the propeller is providing forward thrust. The opposing edge is referred to as the trailing edge, and the radially outer zone extending between the leading edge and the trailing edge is referred to as the blade tip. The root of the blade is the fillet area in the region of transition between the blade surface to the hub periphery.

A few other terms that are commonly used in in propeller nomenclature include:

Diameter: Two times the distance from the center of the hub to the tip of the blade, i.e. equal to the diameter of a circle that the tip of the propeller would make when rotating.

Pitch: Pitch is defined at the theoretical forward movement of a propeller during one revolution, assuming that there is no ‘slippage’ between the propeller blade and the water. In practice some slippage does occur, and the design pitch is therefore more than the actual pitch.

Cupping: Many existing propellers incorporate a cup formation at the trailing edge of the propeller blade. Propeller cup is the deformation of a propeller's trailing edge toward the pressure face. Cupping provides a measure of camber to the blade, and therefore changes the pressure distribution along the blade's chord length, adding lift toward the trailing edge.

Cavitation: Cavitation, (which is often confused with ventilation), is a phenomena of water vaporizing or “boiling” due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller's blade surface due to the collapse of microscopic bubbles on the blade. There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, and physical damage to the blade edges.

Ventilation: Ventilation is a situation where surface air or exhaust gasses are drawn into the propeller blades. When this situation occurs, boat speed is lost and engine RPM climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine.

Hole Shot: Hole shot refers to rapid acceleration of a boat, i.e. from a standing rest or very slow speed until the boat is “on-plane” and riding on top of the water. During this acceleration phase the engine works particularly hard, and it is therefore important to have the best possible propeller for the particular application. A propeller that offers excessive hole shot (i.e. too much acceleration) will typically yield low tops speeds and may cause the engine to operate above its designed RPM. On the other hand, a propeller that offers too little hole shot typically result in poor acceleration performance, and may not bring the engine up to its designed RPM range. Either of these scenarios cause undue strain on the engine and reduces its overall performance and fuel efficiency.

Blade surface area refers to the total surface area of the propeller blade. When a propeller rotates on a fixed axis for any period of time a centrifugal force creating a negative pressure on the blade back of each rotating blade draw water inwards, and when the oncoming blade face comes into contact with the inward flow of water the water is compressed. A positive pressure is therefore induced, and the water in this positive pressure zone then exerts a force against the adjacent body of water, resulting in thrust. Standard blade designs allow the inward flow of water to flow over the entire curvature of the blade back. This is believed to result in about 40% of the energy being wasted because on average only 60% of the negative pressure water mass is compressed by the blade face of an oncoming blade. Further energy is lost between the blade roots of each blade back, which fragments the flow of water when the positive pressure water mass collides with the negative pressure water mass. This disturbance affects the volume of water mass that gets displaced.

It will be appreciated that blade surface area plays an important part in propeller performance. This is important because the more blade surface area a prop has the more water it displaces. However, this can also result in more drag on the blade and thus the engine, due to the effects described hereinbefore. Increased blade area can result in better hole shot, and will allow a boat to remain on plane at lower engine speeds. However, too much blade area can result in excessive drag, and can therefore restrict the RPM that that the engine can develop, and can in addition also result in boat handling issues. The operation of the engine outside its recommended specification results in the engine having to work harder or faster than it is designed to do. This will result in reduced efficiency, fuel economy and possibly also damage to the engine.

It would therefore be beneficial if a propeller can de designed that has a reduced effective blade back area, without reducing the effective area of the blade face.

It is accordingly an object of the invention to provide a propeller that will at least partially alleviate the above disadvantage.

It is also an object of the invention to provide a propeller having reduced drag characteristics without impacting on the surface area of the blade face.

SUMMARY OF THE INVENTION

According to the invention there is provided a propeller including:

-   -   a hub;     -   a plurality of blades, each blade having a blade face, a blade         back and a blade peripheral zone comprising a leading edge, a         trialing edge and a blade tip zone extending between the leading         edge and the trialing edge;     -   characterized in that a flow guide is provided on the blade back         of each blade.

There is provided for the flow guide to be in the form of an elongate ridge extending from a surface of the blade back.

The flow guide may be linear, and may be angularly offset relative to a radially outwardly direction of the propeller.

Preferably the flow guide is at least partially parallel to a trailing edge of the blade.

Preferably the flow guide is located closer to the trialing edge of the blade than to the leading edge of the blade.

There is provided for the flow guide to be of a tapering profile when viewed in cross-section.

Preferably a side of the flow guide disposed towards the trialing edge of the blade is higher than a side disposed towards a leading edge of the blade.

An upper surface of the flow guide may be disposed at an angle of between 10 and 40 degrees relative to the surface if the blade back. Preferably, the angle is between 25 and 35 degrees.

There is provided for the flow guide to extend from the blade root towards at least halve the distance to the periphery of the blade.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of a non-limiting example, and with reference to the accompanying drawings in which:

FIG. 1 is a front perspective view of the propeller including flow guides in accordance with one embodiment of the invention;

FIG. 2 is a side view of the propeller of FIG. 2;

FIG. 3 is top plan view of the propeller of FIG. 1.

DETAILED DESCRIPTION OF INVENTION

Referring to the drawings, in which like numerals indicate like features, a non-limiting example of a propeller in accordance with the invention is generally indicated by reference numeral 10.

The propeller 10 comprises a hub 20, which may be of many different configurations. In one embodiment the hub 20 incorporates a secondary propulsion system as described in the applicant's co-pending application ZA2012/05757 entitled “Propeller incorporating a secondary propulsion system”, the contents of which is incorporated herein by reference. In one embodiment the propeller also includes the provision of edge members on the blades of the propeller as described in the applicant's co-pending application ZA2012/05753 entitled “Propeller including a discrete edge member”, the content of which is also incorporated herein by reference.

A plurality of blades 30 extend radially outwardly from the hub 20, with each blade being in the form of an airfoil extending from the hub 20 at a root 23 section thereof, and terminating in a peripheral tip zone 34. The blade 30 includes a blade face 31 and a blade back 32. The periphery of the blade 30 comprises a leading edge 35, a trailing edge 36, and an outer tip zone 34 extending between the leading edge 35 and the trailing edge 36.

Flow guides 50 in the form of elongate ridges are provided on the surface of the blade backs 32. Each flow guide 50 is in the form of a linear ridge that is somewhat angularly offset relative to a radially outwardly direction of the propeller. More particularly, each flow guide is at least partially aligned and parallel relative to a trialing edge of the blade 30. The flow guide 50 is disposed towards a proximal zone of the blade surface 32, with a first end 50.1 of the flow guide being located adjacent the hub 20 of the propeller 10, and with a second end 50.2 extending towards the periphery of the blade.

In cross-section, the flow guide 50 is of a tapered configuration, with a first side 50.3, which is the side facing the leading edge of the blade, being flush with the surface of the blade back 32, but with an opposite side 50.4, which is the side facing the trialing edge of the blade, being raised relative to the surface of the blade back. The gradient between the first side 50.3 and the second side 50.4, and hence the upper surface of the flow guide, is linear in this particular embodiment, and more particularly forms an angle of about 30 degrees relative to the blade back 32. The gradient and hence upper surface may however vary in terms of profile and magnitude, and may for example also be arcuate, and in particular somewhat convex.

The flow guide has the effect of directing incoming water away from the blade back and towards the oncoming blade face. The incoming water therefore only flow across between 40% and 60% of the back blade before it is redirected towards the oncoming blade. This results in an increase efficiency accompanied by a reduction in drag, as is discussed in detail in the background to this invention.

The propeller 10 body is made from magnesium or a magnesium alloy, which is made in a moulding process known in the art. The flow guides 50 are integrally formed with the propeller blades and body.

It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention. 

1-11. (canceled) 1-12. (canceled)
 13. A propeller including: a hub; and a plurality of blades, each blade having a blade face, a blade back and a blade peripheral zone comprising a leading edge, a trialing edge and a blade tip zone extending between the leading edge and the trialing edge; wherein the blade face is substantially smooth, and a flow guide is provided on the blade back of each blade in order for incoming water to flow across a part of the blade back before it is redirected towards a blade face of an oncoming blade.
 14. The propeller of claim 13, wherein the flow guide is in the form of a single elongate ridge protruding from a surface of the blade back.
 15. The propeller of claim 14, wherein the flow guide is spaced apart from the leading edge of the blade at a distance of between 40% and 60% of the width of the blade, with the blade back surface between the flow guide and the leading edge being substantially smooth.
 16. The propeller of claim 13, wherein the flow guide is linear.
 17. The propeller of claim 16, wherein the flow guide is angularly offset relative to a radially outwardly direction of the propeller blade.
 18. The propeller of claim 17, wherein the flow guide is at least partially parallel to a trailing edge of the blade.
 19. The propeller of claim 13, wherein the flow guide is located closer to the trialing edge of the blade than the leading edge of the blade.
 20. The propeller of claim 13, wherein the flow guide is of a tapering profile when viewed in cross-section.
 21. The propeller of claim 20, wherein a side of the flow guide disposed towards the trialing edge of the blade is higher than a side disposed towards a leading edge of the blade.
 22. The propeller of claim 21, wherein an upper surface of the flow guide is disposed at an angle of between 10 and 40 degrees relative to the surface of the blade back.
 23. The propeller of claim 22, wherein an upper surface of the flow guide is disposed at an angle of between 25 and 35 degrees relative to the surface of the blade back.
 24. The propeller of claim 13, wherein the flow guide extends from the blade root towards at least half the distance to the periphery of the blade. 